1. Field of the Invention
The invention relates to a mercury discharge lamp of the short arc type which is used for a light source of an exposure device for semiconductor exposure and for liquid crystal exposure.
2. Description of Related Art
Conventionally, in a mercury discharge lamp of the short arc type in an arc tube composed of a quartz bulb, there is a pair of electrodes, i.e., a cathode and an anode, whereby a distance between the electrodes is maintained at 2 mm to 13 mm, the arc tube being filled with the combination of mercury and a rare gas. The amount of mercury added to the arc tube is determined in accordance with the demand for light emitted by the lamp. When line spectra with a narrow band are required, as in semiconductor exposure, the amount of mercury to be added is reduced. When spectra having a large bandwidth are required, as in liquid crystal exposure, the amount of mercury to be added is increased.
The rare gas added to the arc tube is used mainly to improve the lamp starting property. But there are also cases in which it also has the purpose of increasing the heat insulation effect within the arc tube and the radiant efficiency. In one such mercury discharge lamp of the short arc type, however, there have been cases in which a phenomenon occurred such that devitrification or breakage of the arc tube took place within a short time immediately after the start of operation.
The reason for this phenomenon is described using FIG. 8. In a mercury discharge lamp of the short arc type, the pressure within the arc tube 1 before operation corresponds roughly to the pressure of the rare gas and is much lower than the pressure during operation. The principle reason for this is that before operation, the vapor pressure of the mercury is low. The reason for adding rare gas to the arc tube 1 is to reduce the discharge breakdown voltage between the cathode 2 and the anode 3 and to enable easier starting of the discharge.
In a state in which the pressure in the arc tube 1 is low directly after a discharge breakdown, the average free path length of the electrons released from the cathode 2 into the discharge space becomes greater. These electrons are in a state in which they can fly not only easily in the direction towards the anode 3, but also in the direction towards the arc tube 1.
As a result, there is an area (hereinafter called the xe2x80x9ccharge area Rxe2x80x9d) in which the electrons which have a few collisions behind themselves and which have flown into the vicinity of the arc tube 1 ionize the rare gas in the vicinity of the electrons, and thus, in the vicinity of the inside of the arc tube 1. In this way, the electrons are attracted one after the other in the direction to this charge area R and move there. The arc due to the discharge which forms between the electrodes is maintained for a while after operation in a state away from the electrode axis X, i.e., in a state in which it has approached the surface of the arc tube 1 or is in contact with it. As a result, there have been cases in which devitrification of the arc tube 1 occurred and in which the arc tube 1 eventually broke.
The present invention is provided in order to eliminate the aforementioned disadvantages in the related art. Accordingly, the object of the present invention is to provide a mercury discharge lamp of the short arc type in which a discharge which forms between the electrodes when the lamp starts is formed reliably along the electrode axis, and thus, the arc is reliably formed and maintained along the electrode axis in order to prevent devitrification and breakage of the arc tube.
A mercury discharge lamp of the short arc type includes an arc tube filled at least with a combination of mercury and a rare gas, and a pair of electrodes, i.e., a cathode and an anode, positioned oppositely therein. In accordance with the present invention, the cathode is provided with a tapering area such that in a direction towards the anode an inclined plane is formed.
The object of the invention is achieved in accordance with a first embodiment of the invention in that the amount of mercury added to the arc tube is 0.2xe2x89xa6nxe2x89xa652, where xe2x80x9cnxe2x80x9d is the weight per unit of volume (mg/cm3) of mercury within the arc tube and 0.1xe2x89xa6pxe2x89xa6800, where xe2x80x9cpxe2x80x9d is the pressure (kPa) of the rare gas to be added at an ambient temperature of 25xc2x0 C., and a projection is formed on the cathode which borders the inclined plane of the tapering area.
The projection which borders the inclined surface of the tapering area of the cathode may be made cylindrical in geometric shape. Moreover, the tip area of the projection on the discharge side is provided with a flat surface, and is also formed bordering the inclined surface of the tapering area of the cathode and the angle between the flat surface and the side of the projection is at most 90xc2x0.